In the oil and gas industry, geophysical survey techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon or other mineral deposits. Generally, a seismic energy source, or “source,” generates a seismic signal that propagates into the earth and is partially reflected by subsurface seismic interfaces between underground formations having different acoustic impedances. The reflections are recorded by seismic detectors, or “receivers,” located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data can be processed to yield information relating to the location and physical properties of the subsurface formations. Seismic data acquisition and processing generates a profile, or image, of the geophysical structure under the earth's surface. While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of them. Thus, providing a high-resolution image of the subsurface is an ongoing process.
Various sources of seismic energy have been used to impart the seismic waves into the earth. Such sources have included two general types: 1) impulsive energy sources and 2) seismic vibrator sources. The first type of geophysical prospecting utilizes an impulsive energy source, such as dynamite or a marine air gun, to generate the seismic signal. With an impulsive energy source, a large amount of energy is injected into the earth in a very short period of time. In the second type of geophysical prospecting, a vibrator is used to propagate energy signals over an extended period of time, as opposed to the near instantaneous energy provided by impulsive sources.
The seismic process employing such use of a seismic vibrator, sometimes referred to as “vibroseis,” propagates energy signals into the earth over an extended period of time. The data recorded in this way is then correlated to convert the extended source signal into an impulse. In land-based implementations. the source signal is generally generated by a servo-controlled hydraulic vibrator, or “shaker unit,” mounted on a mobile base unit. In marine implementations, vibrators typically include a bell-shaped housing with a large and heavy diaphragm in its open end. The vibrator is lowered into the water from a marine survey vessel, and the diaphragm is vibrated by a hydraulic drive system similar to that used in a land vibrator. Except where expressly stated herein, “source” is intended to encompass any seismic source implementation, both impulse and vibratory, including any dry land or marine implementations thereof.
Vibrators typically employ a sweep generator to control the vibratory source signal emitted by the vibrator. The output of the sweep generator dictates the manner in which the frequency of the emitted signal, which is imparted into the earth, varies as a function of time. Typically, the impartation of energy with a vibrator is for a preselected time interval. The vibrator radiates energy at varying frequencies into the earth's crust during the preselected time interval or “sweep.” In such instances, energy at a starting frequency is first imparted into the earth, and the vibration frequency changes over the sweep interval at some rate until the stopping frequency is reached at the end of the interval. The difference between the starting and stopping frequencies of the sweep generator is known as the range of the sweep, and the amount of time used to sweep through those frequencies is known as the sweep time.
A sweep may have various characteristics. A sweep may be linear such that the frequency changes linearly over the sweep time at a rate dictated by the range of the sweep and the sweep time. In contrast, a sweep may be nonlinear such that the frequency varies nonlinearly between the starting and stopping frequencies over the sweep time. For example, in conventional systems, a nonlinear sweep may include a quadratic sweep or a logarithmic sweep. A sweep is continuous such that the source generates substantially all frequencies between the starting and stopping frequency.
Seismic sweeps often have durations between two and twenty seconds. The instantaneous frequency of the seismic sweep may vary linearly or nonlinearly with time. The instantaneous frequency is the time derivative of the instantaneous phase of the seismic signal. The ratio of the instantaneous frequency variation over the unit time interval is defined as the sweep rate. Further, the frequency of the seismic sweep may start at a low frequency and increase with time, called “an upsweep,” or it may begin at a high frequency and gradually decrease, known as “a downsweep.” Typically, the frequency range is from about two Hertz (Hz) to an upper limit that is often less than two-hundred Hz, most commonly about one-hundred Hz. In the case of sweep usage, the recorded signal is correlated with the emitted sweep in order to retrieve a wavelet form providing the time information.
The seismic signal is emitted in the form of a wave that is reflected off interfaces between geological layers. Typical seismic exploration may involve the creation of two types of source waves: surface waves and body waves. Surface waves are the waves that travel along the earth's surface when the seismic energy signal is emitted. Body waves travel into the interior of the earth and generate the reflected waves, such as P-waves (primary waves) and S-waves (secondary waves). A P-wave is the most commonly used form of seismic wave. P-waves cause particles to oscillate parallel to the direction in which the wave propagates. An S-wave, generated by most land seismic sources and sometimes by converted P-waves, is a wave in which particles oscillate perpendicular to the direction in which the wave propagates. In some cases, S-waves can be converted to P-waves. When the wave encounters an interface between different media in the earth's subsurface a portion of the wave is reflected back to the earth's surface while the remainder of the wave is refracted through the interface.
The reflected waves are received by an array of geophones, or receivers, located at the earth's surface, which convert the displacement of the ground resulting from the propagation of the waves into an electrical signal recorded by means of recording equipment. The receivers typically record data during the source's sweep interval and during a subsequent “listening” interval. The receivers record the time at which each reflected wave is received. The travel time from source to receiver, along with the velocity of the source wave, can be used to reconstruct the path of the waves to create an image of the subsurface.
A large amount of data may be recorded by the receivers and the recorded signals may be subjected to signal processing before the data is ready for interpretation. The recorded seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations. That information is then used to generate an image of the subsurface.
As described above, in conventional systems, a seismic signal is emitted featuring a continuous spectrum, emitting all frequencies. The emitted signal may cover a large bandwidth to enhance wavelet quality and improve geophysical inversion modeling results. However, there may be some frequencies, referred to as noise-susceptible frequencies, within the sweep range where the signal-to-noise ratio of the reflected signal may be low and may create a lack of information for signal processing use in those noise-susceptible frequency bands. When the signal-to-noise ratio of the reflected signal is unusable the data processing system will lack information to enable imaging of the subsurface formations at noise-susceptible frequencies. Further, seismic exploration efficiency is reduced when the source emits a signal that does not return a usable reflected signal. Accordingly, it would be desirable to provide systems and methods that do not emit a seismic signal at noise-susceptible frequencies and improve the seismic exploration efficiency to avoid the previously described problems and drawbacks.